The present invention relates to highly active catalysts for olefin metathesis reactions, and the preparation of the catalysts. The invention also relates to the olefin metathesis reactions catalyzed with the catalysts of the invention.
A number of catalysts have been developed recently for initiating olefin metathesis reactions, including ring-opening metathesis polymerization (ROMP) of cyclo-olefins, ring-closing metathesis (RCM) of dienes to form ring-closed products, depolymerization of unsaturated polymers to form the depolymerized products, synthesis of telechelic polymers by reaction of a cyclic olefin with a functionalized olefin, and synthesis of cyclic olefins by self-metathesis of an acyclic olefin or cross-metathesis of two acyclic olefins. Those well defined catalysts usually have a metal-carbon double bond (metal-carbene or -alkylidene) that can coordinate to the alkene moiety of the olefin and, for example, can initiate readily the ring opening of cyclo-olefin monomers. Most of the metals that exhibit remarkable activity in such catalysts are second-or third-row mid-to late- transition metals. Although the specific reason for their degree of activity has not been clearly established, many theories have been put forward, the most prevalent of which expounds that late transition metals exhibit greater robustness towards the impurities that may inherently be present within a reaction system and consequently resist degradation.
Among olefins, cyclic olefins like norbornene (NB) or endo-dicyclopentadiene (DCPD) which possess a strained double bond can readily undergo ring opening metathesis polymerization (ROMP) because the ring opened product is thermodynamically favored. The above-mentioned catalysts are particularly active in catalyzing the ROMP of such ring-strained cyclo-olefins.
The catalysts that have received the greatest exposure in the literature by far are those designed by Schrock et al., as reported in Schrock et al., J. Am. Chem. Soc., 1990, 112, 3875, and by Grubbs's group, as reported in Nguyen et al., J. Am. Chem. Soc., 1993, 115, 9858; Nguyen et al., J. Am. Chem. Soc., 1992, 114, 3974; and Grubbs et al., WO98/21214 (1998). The Grubbs catalyst (a ruthenium carbene) is slightly more versatile than the Schrock catalyst (a molybdenum alkylidene) because of its ease of synthesis as well as its utility from a commercial viewpoint. Recently, Cox and co-workers reported in Cox et al., Inorg. Chem., 1990, 29, 1360; Cox, et al., J. Chem. Soc., Chem. Commun., 1988, 951-953; and Porri et al, Tetrahedron Letters, No. 47., 1965, 4187-4189, the synthesis of a class of metal catalysts based on ruthenium metal. These catalysts consist primarily of a bis-allyl ligand wrapping the metal, along with two or three acetonitrile ligands. Additionally, these catalysts possess a mono- or di-anion that is virtually (i.e., almost) coordinated to the metal center, which is therefore considered to be formally in the +4 oxidation state. These complexes in conjunction with a diazo ethyl acetate have been used by Herrmann's group, as reported in Herrmann et al., Angew. Chem. Int'l. Ed. Engl., 1996, 35, 1087, to investigate the polymerization (specifically the ROMP) of NB. Herrmann has conjectured that the active species in the catalyst system is a metal carbene generated in situ when the ruthenium reacts with the diazo alkyl compound (such as diazo ethyl acetate).
A disadvantage of the above catalysts is that for the ROMP of cyclic olefins these catalysts must be used with a co-catalyst such as a diazo alkyl compound, which requires special caution in handling because of the instability of the diazo group.